The LTER Program and Climate

Both ecologists and climatologists recognize climate research as having a key role in long-term ecological research. Climate is one of the largest driving forces of ecological and hydrological processes at all of the LTER sites. Each LTER site is required to organize its 6-year research program around a central fundamental working hypothesis. A majority of the sites have climate as a central component of their research hypothesis. For example, one of the central questions of the H. J. Andrews Experimental Forest LTER research is, How do land use, natural disturbance, and climate change affect three key ecosystem properties: carbon dynamics, biodiversity, and hydrology? The goals of the Arctic LTER Project are to understand how tundra, streams, and lakes function in the Arctic and to predict how they respond to changes, including changes in climate. It is therefore essential to investigate the climate of the LTER sites in a systematic manner. Each LTER site maintains its own climate program and, at many sites, climate data represent the longest time sequence of data available. Increasing attention to possible ecological consequences of global climate change requires that we understand how climate varies and what the potential is for rapid directional climate change (LTER 1989; Greenland and Swift 1990 and 1991; IPCC 2001).

An example of the importance of long-term climate, or climate-related, information to ecosystem science may be taken from an aquatic LTER site. The number of days of ice cover on Lake Mendota, Wisconsin, which is part of the North Temperate Lakes (NTL) LTER site, illustrates the importance of long-term records and the need for benchmark climatic studies (Magnuson 1990; Robertson et al. 1992; Magnuson et al. 2000). If one started observing in 1998, one might conclude there are about 50 days of ice cover on the lake. However, the data for the decade 1989-1998 indicate that the average length of ice cover was about 100 days and that the 1998 value was "unusual." Fifty years of data (1949-1998) show a downward trend from about 110 to 90 days, with El Niño years having very short values of ice cover, as in 1998. The complete observed record starting in 1856 confirms the downward trend in the number of ice cover days as well as suggests interesting in-terdecadal variability. The duration of ice cover in this aquatic ecosystem determines the productivity and activity at all trophic levels during the ice-free summer period.

Although many of the analyses presented in this volume could be made with any subset of data from U.S. climate stations or climate divisions, there are specific rea-

Mediterranean

Polar Ice Cap Polar Marine sons for concentrating on LTER sites. First, the analyses are directly focused on the LTER sites that have a legacy of ecosystem research. Second, the sites have ongoing, coherent programs of ecosystem research. Third, several of the LTER sites have climate stations at places rarely sampled by national weather observing systems. The alpine tundra NWT D1 site at an elevation of 3749 m (12,300 ft.) is a case in point.

It is helpful to pause and reflect on exactly what the "climate" in climate variability and ecosystem response actually is. This question is raised by Goodin et al. (chapter 20) for the context of Net Primary Productivity (NPP) at the Konza Prairie. In this specific context the "climate" has been defined using values of air temperature, precipitation, and pan evaporation with various indexes derived from these variables, while bearing in mind subsets of time such as the "growing season." We use the term climate differently for almost every different ecosystem considered in this book. The climate that ecosystems experience is most truly represented by values of heat, moisture, gas, and momentum exchange at what the Russian scientist Alexander I. Voeikov called in 1884 the "outer effective surface" of the ecosystem components. Except in cases of the most detailed microclimatological studies, ecologists and climatologists usually deal with values of variables such as air temperature and precipitation that act only as surrogates of the variable that we ought to be measuring. Thus we see "through a glass darkly." This approach is forced on us partly by practical and economic considerations and partly because most meteorological observing networks are established with weather forecasting rather that climate/ecosystem interaction purposes in mind.